Upgrade Windows 8 Release Preview To RTM

No. It is not possible to upgrade from Windows 8 Release Preview to RTM. The error message is:

Compatibility report (saved to your desktop)

You can’t upgrade this prelease version of Windows. Go to http://go.microsoft.com/fwlink/?LinkID=142337 to see how to install Windows and keep your files and settings.

Upgrade Windows 8 Release Preview To RTM

You don’t have to delete the partition Windows 8 Beta is on. When installing the Windows 8 RTM it will move the old windows installation into a folder called “windows.old”. So you don’t have to format your partition. The folder and files will be preserved.

Make a fresh installation. Further details have a look here